Add note of separate compilation unit in macro's code example

Author: mentegy

docs/docs/reference/principled-meta-programming.md

@@ -321,7 +321,8 @@ program that calls `assert`.
       def assertImpl(expr: Expr[Boolean]) =
         '{ if !(~expr) then throw new AssertionError(s"failed assertion: ${~expr}") }
     }
-
+    
+    // has to be in different compilation unit
     val program = {
       val x = 1
       Macros.assert(x != 0)